1. Field of the Invention
The present invention relates to a failure diagnosis apparatus for a temperature sensor. This temperature sensor detects a temperature that changes depending on an operating condition of an internal combustion engine.
2. Description of the Related Art
A failure-diagnosis apparatus for a temperature sensor, which detects an engine cooling water temperature that changes depending on an operating condition of an internal combustion engine, is shown in Japanese Patent Laid Open No. 2003-20988. According to the apparatus disclosed in Japanese Patent Laid-Open No. 2003-20988, a temperature difference between a stop temperature detected at the end of the preceding engine operation (at the time the engine was stopped) and a start temperature detected at the beginning of the present engine operation is compared with a predetermined determination value. If the temperature difference is less than or equal to the predetermined determination value, it is determined that the temperature sensor fails. This determination is made on condition that a value of an engine operation duration parameter, such as a fuel consumption amount at the end of the preceding engine operation, is greater than or equal to a predetermined value, and an engine stop period immediately before the beginning of the present engine operation is greater than a predetermined time period.
In an example where the engine cooling water temperature TW changes as shown in FIG. 10, a maximum change amount of the cooling water temperature TW is the difference (TW1−TW3) between a cooling water temperature TW1 at time t21 and a cooling water temperature TW3 at time t23 (at the time of starting). However, in the above-mentioned conventional apparatus, a failure determination is performed using a difference (TW2−TW3) between a cooling water temperature TW2 at time t22 (at the time of engine stoppage) and the cooling water temperature TW3 at time t23 (at the time of engine starting). Therefore, the temperature difference (TW2−TW3) is smaller than the temperature difference (TW1−TW3). Consequently, the failure determination accuracy may sometimes become lower.
Further, as to the engine having a block heater, if the atmospheric temperature greatly falls after engine stoppage, the block heater may be turned on and the cooling water temperature may sometimes rise after engine stoppage. Therefore, the cooling water temperature detected at the time of engine starting may become higher than usual, a temperature difference between a cooling water temperature upon engine stoppage and a cooling water temperature upon engine starting may become small, and the failure determination accuracy may become lower.